17.1 Physical Defenses

Mechanical defenses are physical barriers and processes that prevent the entry or facilitate the removal of pathogens from the body. These include structures like skin and mucous membranes, as well as actions like coughing and sneezing.

Skin: A physical barrier composed of multiple layers that prevents pathogen entry.

Mucus: A thick fluid secreted by mucous membranes that traps pathogens.

Ciliary Escalator: The coordinated movement of cilia in the respiratory tract to expel mucus and trapped particles.

Mucus is a viscous secretion produced by mucous membranes that serves as a protective barrier and trap for pathogens. It plays a critical role in the immune responses of the respiratory, digestive, and other systems.

Lysozyme: An enzyme found in mucus that breaks down bacterial cell walls.

Goblet Cells: Specialized epithelial cells that secrete mucus.

Antimicrobial Peptides: Small proteins found in mucus that have broad-spectrum antimicrobial activity.

Cilia are small, hair-like projections that extend from the surface of certain cells, primarily those found in the respiratory and reproductive systems. These structures play a crucial role in the physical defenses of the body by facilitating the movement of materials and particles across cell surfaces.

Ciliary Action: The coordinated, wavelike movement of cilia that propels materials, such as mucus and foreign particles, across cell surfaces.

Mucociliary Escalator: The mechanism by which cilia in the respiratory tract work in conjunction with mucus to trap and remove particles, microorganisms, and other foreign materials from the airways.

Epithelial Cells: The cells that line the surfaces of the body, including the respiratory and reproductive systems, and often possess cilia to facilitate movement and transport.

The microbiome refers to the diverse community of microorganisms, including bacteria, archaea, viruses, and fungi, that reside in a particular environment, such as the human body. These microorganisms form complex, symbiotic relationships with their host, playing crucial roles in various aspects of health and disease.

Microbiota: The collection of microorganisms, primarily bacteria, that inhabit a specific environment, such as the human gut or skin.

Symbiosis: The close and often long-term interaction between different organisms, where both species benefit from the relationship.

Dysbiosis: An imbalance or disruption in the normal composition and diversity of the microbiome, which can contribute to the development of various health conditions.

Keratin is a fibrous structural protein that is the primary component of the outer layer of human skin, hair, and nails. It provides strength, flexibility, and protection to these tissues.

Epithelial Tissue: The type of tissue that forms the outer layer of the skin and lines various organs and body cavities. Keratin is a key structural component of epithelial tissues.

Intermediate Filaments: Rope-like protein structures that provide mechanical support and shape to cells, including the keratin filaments found in epithelial cells.

Desquamation: The natural shedding of the outermost layer of the skin, which is composed primarily of dead, keratinized cells.

Sebaceous glands are small oil-producing glands present in the skin. They secrete sebum, which lubricates and waterproofs the skin and hair.

Acne: A common skin condition resulting from blocked or inflamed sebaceous glands.

Sebum: An oily substance secreted by sebaceous glands to lubricate and protect the skin.

Microbiota: The community of microorganisms living on body surfaces including skin, contributing to health and disease prevention.

Sebum is an oily substance produced by sebaceous glands in the skin. It plays a role in maintaining skin hydration and providing a barrier against microbial invasion.

Lipids: Organic compounds that are fatty acids or derivatives, insoluble in water but soluble in organic solvents.

Innate Immunity: The body's first line of defense against pathogens, consisting of physical barriers, chemical barriers, and immune cells.

Sebaceous Glands: Small oil-producing glands present in the skin connected to hair follicles that secrete sebum.

Mucous membranes are moist, protective linings that cover the body's internal surfaces, such as the nose, mouth, throat, lungs, and gastrointestinal tract. They play a crucial role in the body's physical defenses against pathogens and other harmful substances.

Goblet Cells: Specialized cells within the mucous membranes that secrete mucus, a sticky, viscous substance that traps and removes foreign particles, microbes, and other contaminants.

Cilia: Tiny, hair-like projections on the surface of some mucous membrane cells that move in a coordinated fashion to propel mucus and trapped particles out of the body.

Mucus: A sticky, viscous substance produced by goblet cells and other specialized cells within the mucous membranes that traps and removes foreign particles, microbes, and other contaminants from the body.

Mucus is a viscous secretion produced by mucous membranes that serves as a protective barrier and trap for pathogens. It plays a critical role in the immune responses of the respiratory, digestive, and other systems.

Lysozyme: An enzyme found in mucus that breaks down bacterial cell walls.

Goblet Cells: Specialized epithelial cells that secrete mucus.

Antimicrobial Peptides: Small proteins found in mucus that have broad-spectrum antimicrobial activity.

Lysozyme is an enzyme that breaks down the cell walls of bacteria, particularly Gram-positive bacteria, by cleaving the bonds in peptidoglycan. It is a crucial component of the innate immune system and is found in various body fluids such as tears, saliva, and mucus.

Peptidoglycan: A polymer that forms a protective layer outside the plasma membrane of most bacteria.

Innate Immunity: The non-specific first line of defense against pathogens present at birth.

Gram-Positive Bacteria: Bacteria with thick peptidoglycan layers in their cell walls that retain crystal violet stain during Gram staining.

Peristalsis is the rhythmic contraction and relaxation of muscles in the gastrointestinal tract that moves food through the digestive system. It also helps expel pathogens and debris from the body.

Mucociliary Escalator: A mechanism involving cilia and mucus to trap and move particles out of the respiratory tract.

Gastrointestinal Tract: The pathway from the mouth to the anus where digestion and absorption occur.

Autonomic Nervous System: The part of the nervous system responsible for controlling involuntary bodily functions, including peristalsis.

Probiotics are live microorganisms, typically bacteria, that are beneficial to human health, especially the digestive system. They are often referred to as the 'good' bacteria that can help maintain a healthy gut microbiome, support immune function, and promote overall wellbeing.

Gut Microbiome: The diverse community of microorganisms, including bacteria, viruses, and fungi, that reside in the human gastrointestinal tract and play a crucial role in various physiological processes.

Prebiotics: Non-digestible food ingredients that selectively stimulate the growth and activity of beneficial gut bacteria, such as probiotics.

Dysbiosis: An imbalance in the composition and diversity of the gut microbiome, which can contribute to various health issues.

Commensals are microorganisms that live in close association with a host, such as the human body, without causing harm or providing any significant benefit to the host. They are part of the normal microbiota and coexist peacefully, occupying specific niches within the host environment.

Microbiome: The collective genome of the microorganisms residing in a particular environment, such as the human body.

Mutualism: A symbiotic relationship where both organisms involved benefit from the interaction.

Pathogen: A microorganism capable of causing disease in a host.

Bacteriocins are antimicrobial peptides or proteins produced by bacteria that can inhibit or kill closely related bacterial strains. They play a role in bacterial competition and defense within microbial communities.

Antimicrobial Peptides: Small, naturally occurring molecules that can disrupt the membranes of microorganisms and inhibit their growth.

Microbial Interactions: The complex relationships and communication between different microorganisms within a shared environment or community.

Quorum Sensing: A process by which bacteria can detect and respond to cell density by producing and sensing signaling molecules.

Colicins are protein toxins produced by certain strains of Escherichia coli bacteria. They act as a defense mechanism, allowing the producing bacteria to inhibit or kill closely related competitor strains, thereby providing a competitive advantage in the environment.

Bacteriocins: Bacteriocins are antimicrobial proteins produced by bacteria that can inhibit or kill other bacteria, often closely related strains.

Plasmids: Plasmids are small, circular DNA molecules found in bacteria that can carry genes for colicin production and immunity.

Receptor-Mediated Uptake: The process by which colicins bind to specific receptors on the target bacterial cell surface and are transported into the cell, leading to cell death.

Nisin is a polypeptide antimicrobial compound produced by certain strains of the Gram-positive bacterium Lactococcus lactis. It is classified as a bacteriocin and is primarily used as a food preservative due to its ability to inhibit the growth of many Gram-positive bacteria, including Listeria and Clostridium species.

Bacteriocin: Bacteriocins are antimicrobial peptides or proteins produced by bacteria that inhibit the growth of closely related bacterial strains.

Listeria: Listeria is a genus of Gram-positive bacteria that can cause the foodborne illness listeriosis, particularly in pregnant women, newborns, and individuals with weakened immune systems.

Clostridium: Clostridium is a genus of Gram-positive, spore-forming bacteria that can cause various diseases, including botulism, tetanus, and gas gangrene.

Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system by recognizing molecules frequently found on pathogens. They help initiate immune responses and bridge innate and adaptive immunity.

Major Histocompatibility Complex (MHC): A set of cell surface proteins essential for acquired immune system to recognize foreign molecules.

Antigen-Presenting Cells (APCs): Cells that display antigens complexed with MHCs on their surfaces for recognition by T cells.

Cytokines: Small proteins important in cell signaling released by cells, especially those in the immune system.

TH17 cells are a subset of CD4+ T helper cells that produce the cytokine IL-17 and play a crucial role in defending against extracellular pathogens and contributing to inflammatory responses.

CD4+ T Cells: A type of immune cell that assists other white blood cells in immunologic processes.

Cytokine IL-17: A pro-inflammatory cytokine produced by TH17 cells involved in host defense and autoimmunity.

TGF-β (Transforming Growth Factor-beta): A multifunctional cytokine that plays a crucial role in immune regulation and differentiation of various T cell subsets including TH17.

The hygiene hypothesis suggests that a lack of early childhood exposure to infectious agents, symbiotic microorganisms, and parasites increases susceptibility to allergic and autoimmune diseases by preventing the normal development of the immune system. It proposes that our modern, overly hygienic environments deprive the immune system of necessary stimuli, leading to dysregulation and an increased risk of immune-related disorders.

Immune System Development: The process by which the immune system matures and learns to recognize self from non-self, allowing it to mount appropriate responses to pathogens while avoiding autoimmunity.

Autoimmune Diseases: Conditions where the immune system mistakenly attacks the body's own healthy tissues, leading to chronic inflammation and organ damage.

Allergic Diseases: Hypersensitive immune responses to otherwise harmless environmental triggers, such as pollen, dust mites, or certain foods.

The skin barrier is the outermost layer of the skin that acts as a physical and chemical defense against pathogens. It prevents harmful microorganisms from entering the body and helps maintain overall skin health.

Stratum Corneum: The outermost layer of the epidermis made up of dead keratinized cells.

Antimicrobial Peptides: Small proteins produced by epithelial cells that kill or inhibit microbial pathogens.

Sebum: An oily substance produced by sebaceous glands that helps protect and lubricate the skin.

The stratum corneum is the outermost layer of the epidermis, composed mainly of dead keratinized cells. It acts as a barrier to protect underlying tissues from infection, dehydration, and physical harm.

Epidermis: The outermost layer of skin consisting of multiple sublayers including the stratum corneum.

Keratinocytes: Cells that produce keratin and make up most of the epidermal cells.

Sebum: An oily substance produced by sebaceous glands that helps lubricate and protect the skin.

Corneocytes are the flattened, anucleated cells that make up the outermost layer of the skin, known as the stratum corneum. These cells are the end-stage product of keratinocyte differentiation and play a crucial role in the skin's physical defense mechanisms.

Keratinocytes: The predominant cell type in the epidermis, responsible for the production of keratin and the formation of the stratum corneum.

Stratum Corneum: The outermost layer of the epidermis, composed of corneocytes, which provides a physical barrier to protect the body from environmental threats.

Keratinization: The process by which keratinocytes undergo differentiation, ultimately transforming into corneocytes that make up the stratum corneum.

Langerhans cells are a type of dendritic cell found in the epidermis, the outermost layer of the skin. They play a crucial role in the skin's immune defense system and are considered part of the physical defenses against pathogens.

Dendritic Cells: Dendritic cells are specialized antigen-presenting cells that play a central role in the immune system by initiating and regulating immune responses.

Epidermis: The epidermis is the outermost layer of the skin, providing a protective barrier against environmental factors and pathogens.

Antigen Presentation: Antigen presentation is the process by which specialized cells, such as dendritic cells, display foreign antigens on their surface to be recognized by T cells, initiating an immune response.

The acid mantle is a thin, slightly acidic film on the surface of the skin that acts as a protective barrier against environmental stressors and harmful microorganisms. This natural defense mechanism is an important part of the skin's physical defenses.

Stratum Corneum: The outermost layer of the epidermis that forms a protective barrier and helps maintain the acid mantle.

Sebum: The oily substance produced by sebaceous glands that helps maintain the acidic pH of the acid mantle.

Sweat: The secretion from eccrine glands that contains lactic acid and contributes to the acidic pH of the acid mantle.

Sebum is an oily substance produced by sebaceous glands in the skin. It plays a role in maintaining skin hydration and providing a barrier against microbial invasion.

Lipids: Organic compounds that are fatty acids or derivatives, insoluble in water but soluble in organic solvents.

Innate Immunity: The body's first line of defense against pathogens, consisting of physical barriers, chemical barriers, and immune cells.

Sebaceous Glands: Small oil-producing glands present in the skin connected to hair follicles that secrete sebum.

Mucous membranes are moist linings found in various body cavities and passages that open to the external environment. They play a crucial role in trapping pathogens and particulates, thus serving as a first line of defense.

Goblet Cells: Cells within mucous membranes that secrete mucus.

Lysozyme: An enzyme found in mucus that breaks down bacterial cell walls.

Normal Microbiota: The community of beneficial microorganisms residing on mucous membranes and other surfaces of the body.

Epithelial cells are a type of cell that line the surfaces of your body, including skin, blood vessels, organs, and cavities. They form a protective barrier and play a crucial role in innate nonspecific host defenses.

Mucus: A viscous fluid secreted by epithelial tissues that traps particles and microbes.

Cilia: Hair-like structures on certain epithelial cells that move mucus and trapped particles away from sensitive areas.

Antimicrobial Peptides: Small proteins produced by epithelial cells that kill or inhibit microbial growth.

Epithelium is a type of tissue that forms the outer layer of the body and lines the internal organs and cavities. It acts as a protective barrier, facilitating important functions such as absorption, secretion, and sensory perception.

Basal Lamina: A thin, extracellular layer that provides structural support and attachment for the epithelial cells.

Tight Junctions: Specialized cell-to-cell connections that create a barrier to prevent the passage of substances between epithelial cells.

Apical Surface: The outermost surface of an epithelial cell that is exposed to the external environment or lumen of an organ.

Tight junctions are specialized cell-to-cell adhesion complexes that form continuous seals between adjacent epithelial cells. They act as physical barriers, regulating the movement of molecules and ions through the paracellular space, and are crucial in maintaining the integrity of epithelial tissue layers, such as those found in the skin, gastrointestinal tract, and blood-brain barrier.

Adherens Junctions: Adherens junctions are cell-cell adhesion complexes that connect the actin cytoskeletons of adjacent cells, providing structural support and facilitating cell-cell communication.

Desmosomes: Desmosomes are specialized cell-cell junctions that mechanically link the intermediate filament cytoskeletons of neighboring cells, contributing to tissue integrity and strength.

Paracellular Transport: Paracellular transport refers to the movement of molecules and ions through the intercellular spaces between cells, as opposed to transcellular transport, which occurs through the cells themselves.

Claudins are a family of integral membrane proteins that are the main components of tight junctions, which are critical for the formation of epithelial and endothelial cell barriers. They are essential for maintaining the selective permeability of these barriers and regulating the movement of molecules between cells.

Tight Junctions: Tight junctions are specialized cell-cell adhesion complexes that form a selectively permeable barrier between epithelial and endothelial cells, controlling the movement of molecules and ions through the paracellular space.

Epithelial Cells: Epithelial cells are the cells that line the surfaces and cavities of the body, forming a protective barrier against the external environment and regulating the movement of substances across the barrier.

Endothelial Cells: Endothelial cells are the cells that line the interior of blood vessels, lymphatic vessels, and the heart, forming a selectively permeable barrier that controls the movement of substances between the bloodstream and surrounding tissues.

Occludins are transmembrane proteins that play a crucial role in the formation and maintenance of tight junctions between epithelial cells, which are important physical defenses against the entry of pathogens and other harmful substances into the body.

Tight Junctions: Tight junctions are specialized cell-cell adhesion complexes that create a selectively permeable barrier between epithelial cells, regulating the movement of molecules and ions across the epithelium.

Epithelial Cells: Epithelial cells are the cells that line the surfaces and cavities of the body, forming a protective barrier against the external environment.

Paracellular Transport: Paracellular transport refers to the movement of molecules and ions through the spaces between adjacent epithelial cells, as opposed to through the cells themselves.

Goblet cells are specialized epithelial cells that secrete mucus to protect mucous membranes. They play a critical role in trapping pathogens and particles, preventing them from entering the body.

Mucins: Glycoproteins that give mucus its gel-like consistency.

Cilia: Hair-like structures on epithelial cells that move mucus along surfaces.

Epithelial Cells: Cells forming the lining of internal and external surfaces of the body.

Trefoil factors are a family of small, secreted proteins that play a crucial role in maintaining the integrity and repair of mucosal surfaces, particularly in the gastrointestinal tract. These factors are known for their ability to promote cell migration, proliferation, and restitution, which are essential for the protection and regeneration of epithelial tissues.

Mucosal Surfaces: The moist, protective linings of various organs and body cavities, such as the digestive, respiratory, and urinary tracts, which are crucial for maintaining barrier function and preventing infection.

Epithelial Tissues: The thin, protective layer of cells that cover the body's internal and external surfaces, including the skin, gastrointestinal tract, and other organs.

Cell Migration: The directed movement of cells, which is essential for wound healing, immune response, and tissue regeneration.

Lysozyme is an enzyme that breaks down the cell walls of bacteria, particularly Gram-positive bacteria, by cleaving the bonds in peptidoglycan. It is a crucial component of the innate immune system and is found in various body fluids such as tears, saliva, and mucus.

Peptidoglycan: A polymer that forms a protective layer outside the plasma membrane of most bacteria.

Innate Immunity: The non-specific first line of defense against pathogens present at birth.

Gram-Positive Bacteria: Bacteria with thick peptidoglycan layers in their cell walls that retain crystal violet stain during Gram staining.

Secretory IgA (sIgA) is an immunoglobulin found predominantly in mucosal areas, such as the gut, respiratory tract, and urogenital tract. It plays a crucial role in immune function by preventing colonization by pathogens.

Immunoglobulin A (IgA): An antibody that plays a crucial role in mucosal immunity.

Mucosal Immunity: The immune response that protects the body's mucous membranes from infection.

J Chain: A protein component that links two IgA molecules together in secretory IgA.

The lamina propria is a layer of loose connective tissue that underlies the epithelium in various organs and structures, including the gastrointestinal tract, respiratory system, and urinary system. It serves as a supportive framework and contains a variety of cells and structures that contribute to the organ's function and defense.

Epithelium: The thin layer of cells that covers the surface of the body and lines internal organs and cavities.

Connective Tissue: The structural framework that provides support, shape, and connection between different tissues and organs in the body.

Mucosa: The moist, protective layer that lines various internal surfaces of the body, such as the digestive, respiratory, and urinary tracts.

Peyer's patches are organized lymphoid follicles found in the mucosa of the small intestine, particularly the ileum. They play a crucial role in the body's immune defenses and the maintenance of the gut microbiome.

Mucosa-Associated Lymphoid Tissue (MALT): The collection of lymphoid tissues, including Peyer's patches, that are found lining the mucosal surfaces of the body, such as the respiratory and digestive tracts.

Gut-Associated Lymphoid Tissue (GALT): The specialized lymphoid tissues, including Peyer's patches, that are found within the gastrointestinal tract and play a key role in intestinal immunity.

Microfold Cells (M Cells): Specialized epithelial cells found in the follicle-associated epithelium of Peyer's patches that transport antigens from the intestinal lumen to the underlying lymphoid tissue.

M cells, also known as microfold cells, are specialized epithelial cells found in the mucosa-associated lymphoid tissues, such as the Peyer's patches of the small intestine. These cells play a crucial role in the body's physical defenses by facilitating the transport of antigens and microorganisms from the gut lumen to the underlying immune cells.

Peyer's Patches: Peyer's patches are organized lymphoid follicles found in the small intestine that contain a high concentration of M cells, which facilitate the uptake of antigens from the gut lumen.

Antigen Sampling: The process by which M cells transport antigens and microorganisms from the gut lumen to the underlying immune cells, allowing the body to mount an appropriate immune response.

Mucosa-Associated Lymphoid Tissue (MALT): MALT is a collection of lymphoid tissues, including Peyer's patches, that are found in the mucosal linings of the body, such as the gastrointestinal tract, respiratory system, and urogenital tract.

Secretory IgA (sIgA) is an immunoglobulin found predominantly in mucosal areas, such as the gut, respiratory tract, and urogenital tract. It plays a crucial role in immune function by preventing colonization by pathogens.

Immunoglobulin A (IgA): An antibody that plays a crucial role in mucosal immunity.

Mucosal Immunity: The immune response that protects the body's mucous membranes from infection.

J Chain: A protein component that links two IgA molecules together in secretory IgA.

Innate immunity refers to the body's natural, non-specific defense mechanisms that provide immediate protection against pathogens. It is the first line of defense against infectious agents and plays a crucial role in maintaining overall immune function.

Physical Defenses: Physical barriers, such as the skin, mucous membranes, and cilia, that prevent the entry of pathogens into the body.

Cellular Defenses: Immune cells, like phagocytes and natural killer cells, that directly attack and destroy invading microorganisms.

Pathogen Recognition: The process by which the immune system identifies and responds to the presence of foreign substances or pathogens.

Antimicrobial peptides (AMPs) are small proteins that play a crucial role in the innate immune response by directly killing a wide range of pathogens, including bacteria, fungi, and viruses. They are produced by various cells in the body and are particularly abundant in the skin and eyes.

Defensins: A type of antimicrobial peptide that is effective against bacteria, fungi, and many enveloped viruses.

Cathelicidins: Another class of antimicrobial peptides that help protect against bacterial infections and promote wound healing.

Innate Immunity: The body's first line of defense against pathogens, involving mechanisms like physical barriers and antimicrobial peptides.

Biofilms are complex communities of microorganisms, primarily bacteria, that adhere to surfaces and are encased in a self-produced extracellular matrix. These microbial communities exhibit unique characteristics and behaviors that distinguish them from free-floating, planktonic microbes.

Extracellular Polymeric Substances (EPS): The slimy, sticky matrix produced by biofilm-forming microbes that helps them adhere to surfaces and protects the community.

Quorum Sensing: The process by which microbes in a biofilm communicate and coordinate their behavior based on cell density and environmental cues.

Antimicrobial Resistance: The increased tolerance of biofilm-associated microbes to antimicrobial agents, making biofilm infections more difficult to treat.

Mucosal-associated lymphoid tissue (MALT) is a specialized collection of lymphoid tissue that is found in the mucous membranes of the body, such as the gastrointestinal tract, respiratory tract, and genitourinary tract. MALT plays a crucial role in the body's physical defenses against pathogens that attempt to enter through these mucosal surfaces.

Gut-Associated Lymphoid Tissue (GALT): A specific type of MALT found in the gastrointestinal tract, responsible for immune surveillance and protection of the intestinal mucosa.

Bronchus-Associated Lymphoid Tissue (BALT): A type of MALT found in the respiratory tract, providing immune defense against airborne pathogens.

Mucosa: The moist, protective lining of various body cavities and passages, including the digestive, respiratory, and urinary tracts.